NATIONAL WORKSHOP ON FUNCTIONAL MATERlALS 2009 -Conductivity Studies Of Chitosan Based Solid Polymer

Electrolyte Incorporated With Ionic Liquid

M.H. Buraidah, L.P. Teo and A.K. Arof*

Center for Ionics University of Malaya, Department of Physics, Faculty of Science,University of Malaya, 50603 Kuala Lumpur, Malaysia

*Corresponding Author: akaroj@um.edu.my

AbstractChitosan.Nl-lal + I-butyl-3-methylimidazolium iodide [BMIM][I] ionic liquid (IL) basedpolymer electrolyte were prepared by solution cast technique. The ratio of chitosan andammonium iodide (NI-4I) salts was fixed at 55:45 (wt. %). Various concentrations of ionicliquid incorporated into the chitosan.Nllal films have been studied. The highest roomtemperature ionic conductivity was obtained for the sample containing 80 wt. % IL withthe value of 8.47 x 10-4S cm-I. Dielectric properties of this film have been studied in thetemperature range 303 K to 343 K. The AC conductivity was found to increase withincreasing frequency and temperature. The plot of AC conductivity versus frequencyfollows the Jonscher power law <JAC = Aros•

Keywords: chitosan, ionic liquid, polymer electrolyte

1. INTRODUCTION

Ionic liquids are organic salts that exist asliquid at room temperature and containessentially ions. Ionic liquids (ILs) havebecome a part of the "green chemist ry"[1-2] due to unique properties such as non-volatility, non-flammable, high thermalstability and high ionic conductivity [3-7].These unique properties of IL has led toapplications in areas such as chemicalsensor, electrochemical capacitors, polym-er batteries, solar cells, fuel and waterelectrolysis cells and electrochromicsystems [8-13].

In this work, the electrical behavior ofchitosan with ionic liquid has beeninvestigated. Chitosan is a natural polymersynthesized by the deacetylation of chitin[14-15]. Various models such as correlatedbarrier hopping (CBH) model, quantummechanical tunneling (QMT) model andsmall polaron hopping (SPH) model willbe applied to determine the A conductionmechanism for thi ystem.

2. EXPERIMENTAL

The polymer electrolytes were prepared bYsolution cast technique. Chitosan andammonium iodide (NI-4I) salts weredissolved in 50 ml dilute acetic acid withratio 55:45 (wt. %). The solution waSstirred for 24 h at room temperature. Afterthe solution was dissolved in dilute acetiCacid, I-butyl-3-methylimidazolium iodide[BMII] ionic liquid (IL) were added to thesolution and the mixtures were continuov"ly stirred until complete dissolution. Thesolutions were cast in different petri disband allowed to evaporate at ambienttemperature for films to form.

The polymer electrolyte films were cut intOsuitable sizes and sandwiched between twOstainless steel electrodes under springpressure. The impedance of the sample waSdetermined by the complex impedancetechnique using a HIOKI bridge in thefrequency range between 50 Hz to I MWat different temperature.

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3.0 RESULTS AND DISCUSSION

3.1 Conductivity Studies

Fig. 1 shows the Cole-Cole plot (Nquistplot) of chitosan.Nl-lal doped with 60 wt.% IL at selected temperatures. Theconductivity of the electrolyte can becalculated from the equation

(1)

Here, A is the area of the film, t is itsthickness and Rb is bulk resistance. Rb wasobtained from the complex impedance plotat the intersection of the plot and the realimpedance axis. The disappearance ofsemicircle in high frequency regionindicates that the conduction is mainly dueto the ions [16].

100

••0308 K

80 o 313 K o 0

6323 K 6 <>cPx <>

".. 60 x333 K 6<><>c: 6<>'- 0~

40

20

oo 20 40 60 80 100

Z,(Q)

Fig. 2 Impedance plot of (chitosan.Nllal) + 60% IL

Conductivity at room temperature of theelectrolyte with different concentrations ofionic liquid (IL) is shown in Table 1.

Table 1 Conductivity at room temperature for allsample studied

Samples Conductivity, (J ~Scm·l)3.73 x 10'1.27 X 10-61.20 X 10.5

(Chitosan.Nlhl) + 0% IL(Chitosan:N~I) + 20% IL(Chitosan:N~I) + 40% IL

(Chitosan.Nlhl) + 60% IL(Chitosan:N~1) + 80% IL

8.50 X 10.58.47 x 10....

It can be observed that the conductivityincreases with the increase in IL content.The increase in conductivity with ILcontent implies that the number density ofmobile ions, n in the electrolyte and/or themobility of the ions haslhave increasedsince

a = nqJ1 (2)

where n is number density of mobile ions,q is electron charge and J1 is mobility ofions. The highest conductivity can beobserved at 80 wt. % IL with the value of8.47 x 10-4S em". However at this ratio,the polymer electrolyte is not completelydry compared to the film with 40 %(chitosan.Nllal) + 60 % IL that exhibitsconductivity of the order 8.5 x 10.5 S ern".

3.2 Dielectric Studies

The real part of complex permittivity ordielectric constant (er) and dielectric loss(ei), imaginary part of complex permittivitycan be calculated from

(3)

(4)

Here Co=eoAlt, eo is permittivity of freespace and to=Znf, Zi and Z; is theimaginary and real parts of the complexpermittivity, respectively. The dielectricconstant was calculated in the frequencyrange of 50 Hz to 100 kHz. Fig. 2 showsthe angular frequency, to dependence ofdielectric constant (er) for (chitosan.Nllal)+ 60 wt. % IL at selected temperatures. Itcan be observed that e; decreases with theincrease in frequency and is almostconstant at high frequencies. At lowfrequency, electrode polarization isobserved [17-18].

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NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

1400000

1200000 c .303 K0308 K

1000000 + '"313 KtQ 800000 x318 K•

o :.::323K600000 + -328 K

:.:: c .333 K400000 X

.-to+ 338 KI

'" iii200000 o o ~~_3~j•0

4 8 12 16In (J) (Hz)

Fig. 2 Frequency dependence dielectric constantfor (chitosan.Nl-lal) + 60% IL at selectedtemperatures

Fig. 3 shows the temperature dependenceon dielectric constant (&r) at certainfrequencies. It can be seen from the figurethat e; increases with the increase intemperature and it shows strong temperat-ure dependence at lower frequencies.

160000---0 l

140000 02 kHz04 kHz120000 066 kHz

100000 x8 kHz 0 cto" 80000 x 10 kHz

-60 kHz 0 0 660000 .100 kHz 0 0 6 X

40000 0 o 6 X X

0 0 6 X X

0 o 6 ~x20000 0 o i :E

j i i0 .- --r-" .- i300 310 320 330 340 350

T(K)

Fig. 3 Temperature dependence dielectric constantfor (chitosan.Nl-lal) + 60% IL at selected frequenc-ies

Frequency and temperature dependencedielectric loss (z) i shown in Fig. 4 andFig. 5.

1400000 ----------------,

1200000o

.303 Ke 308 K

'"313 Kx 318 Kx323 K

-328 K.333 K

+338 Kc 343 K

1000000~ +

800000 •c

+x C.-+0~~.

o +-------r~

600000

400000

200000

4 8 12In (J)

16

Fig. 4 Frequency dependence dielectric loss for(chitosan.Nllal) + 60% IL at selected temperatures

180000 -----02 kHz 0160000 04 kHz140000 66 kHz 0120000 x8 kHz <>0100000 x 10 kHz

tiS' 0 A-60 kHz 080000 0 6 x.100 kHz n )(x60000 0 c 6 X

0 o l;. x:II:40000 0 0 l;. ~

c o i ~20000 iii - •i i •..0 .. .. . ..

300 310 320 330 340 ,;

T(K)

Fig. 5 Temperature dependence dielectric loss fd(chitosan.Nl'lal) + 60% IL at selected frequencies

It can be observed that e, decreases w~the increase in frequency for a fixtemperature but increases with increasio8temperature for a fixed frequencY'Dielectric loss is also observed to sho'"strong temperature dependence at 10'"frequencies.

3.2 Frequency Dependence AConductivity

Fig. 6 hows the variation of the ACc nductivity (<1AC) with frequency. Froll'the figure it can b ob rved that <1A is

It

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

found to increase with increasingfrequency. The dependence of the ACconductivity on frequency is given byuniversal power law [19-20]:

a AC = alOI + a DC = A(Os (5)

and

In a AC = In A + s In (0 (6)

where aDC is the DC part of the totalconductivity alol' A is a constant, (0 is theangular frequency and s is the power lawexponent with the value in the rangebetween 0 and 1. The value of exponent scan be obtained from the slope InaAC

versus In (0 in the high frequency region if> 28 kHz) where there is no or minimalspace charge polarization.

4,-----------------------5

-6--'s -7~~ -8

~ -9 ...4 '.9 -10 1 ~~, ~ I

~: +------.-~ -~-~ --.---_r- _~ __ I

0303 K 0308 K 6313 K

x 318 K :K 323 K - 328 K

• 333 K + 338 K • 343

4 12 146 8 10

In (J) (Hz)Fig. 6 In UAC versus In OJ for (chitosan:NH4I) + 60%IL at selected temperatures

To determine the conduction mechanism ofmaterials, different theoretical models havebeen developed such as overlapping largepolaron tunneling (OLPT) model, corre-lated barrier hopping (CBH) model, quant-um mechanical tunneling (QMT) modeland small polaron hopping (SPH) model.According to OLPT model [21], the expon-ent s decreases with increasing temperatureto a minimum value then it increases withincreasing temperature. In CBH model

[22], s decreases with increasing temperat-ure but no minimum in s is observed.According to QMT model [23], theexponent s is almost equal to 0.8 andincreases slightly with increasingtemperature or independent ontemperature. In the small polaron (SPH)model [24], s has been observed toincrease with temperature. The temperaturedependence of s for (chitosan.Nl-lal) + 60%IL film is shown in Fig. 7. It can beobserved that s increases with increasingtemperature. This is in good agreementwith the SPH model. Thus the frequencydependence of (lAC can be explained interms of SPH model.

1 r--

0.90.8

0.70.6

~ 0.5

0.4

0.3

0.2••• •••••

•0.:-+-l--~-

300 350310 320 330 340

T(K)Fig. 7 s versus Tfor (chitosan.Nflal) + 60% IL

4. CONCLUSIONS

The electrolyte based on chitosan.Nltaland chitosan.Nl'lal + I-butyl-3-methylimi-dazolium iodide [BMII] ionic liquid (IL)were prepared by solution cast technique.The addition of IL into the electrolyteenhanced the conductivity up to 8.47 X 10-4Scm'] at 80% IL and good solid polymerelectrolytes are obtained until 60% IL.Conduction mechanism of (chitosan.Nl'lal)+ IL sample can be represented by SPHmodel.

ACKNOWLEDGEMENT

M.H. Buraidah thanks Universit; ofMalaya for the SLAB scheme.

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NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

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